Feedwater control system



A. c. wf-:NZEL

FEEDWATER CONTROL SYSTEM Filed NOV. 3, 1948 Patented Apr. Z7, 1954 lUNITED STATS TENT OFFICE FEEDWATER CQN'I'ROL SYSTEM Alfred C. Wenzel, Chicago, Ill., assigner to Republic Flow Meters Company, Chicago, Ill., a

corporation of Illinois 12 Claims.

This invention relates to feedwater control systems, and more particularly to control systems for power plants having a plurality of banks or boilers to control the return ow of feedwater from separate steam consuming devices to the banks of boilers.

In power plants having a plurality of banks of boilers supplying steam to a plurality of steam turbines or other steam consuming units, it is necessary for a balanced operation that feedwater be returned to each of the banks of boilers in proportion to the amount of steam supplied thereby. The term bank of boilers as used herein includes one or more boilers connected in parallel to a common load and having a common feedwater return system. Otherwise, ieedwater may be dumped at one bank and makeup water added at the other resulting in a loss in efliciency. It is the principal object of the present invention to provide a control system which will accomplish this result.

Another object is to provide a control system in which separate feedwater pumps receiving condensate from separate condensers connected to the separate steam consuming devices are controlled-.in response to the ratio of steam flow to eedwater flow from one bank of boilers to the other.

A further object is to provide a control system in which the control is corrected in response to an excess or deciency of :feedwater at one of the return pumps.

The above and other objects and advantages of the invention will be more readily apparent from the following description when read in connection with the accompanying drawing in which the single gure is a diagrammatic illustration Yof a control system embodying the in- `rention.

In the system illustrated, there are two banks of boilers each of which contains two individual boilers, `although it will be apparent that a single boiler or more than two boilers might be contained in each bank. Asshown, one bank contains boilers Eil and II and the other bank contains boilers I2 and'I. The boilers Iii and II supplyV steam through outlet pipes i4 and I5V to a header i5 which is connected at one end to a steam turbine Il. The boilers I2 and I3 supply steam. through outlet conduits I8 and lil to the same header It which is connected at its opposite end to a turbine 2i. With this connection, it will be apparent that the turbine il is principally supplied byl the boilers Hl and I i and that `the turbine 2| is principally suppliedfby the boilers l2 andIS. However, it is possible for steam to flow through the header I6 from one bank of boilers to the other.

In one type of installation, the boilers I0 and II, for example, mightbe olderboilers while boilers I2 and I3 might be newer, more ecient boilers. It is, therefore, desirable in such an .installation to run the boilers I2 andIS at full capacity to supply all of the steam for the turbine EI and a partof the steam for turbine I'I and to runrthe boilers Y Iii and I I only during peak load periods. Other installations of this same general type may be found and may be desirable to meet particular conditions,y and the densed in a condenser 22 and is pumpedby a pump 23 into a deaerator tank 2li. The deaerator tank 24 contains afloat 25 controlling a bypass valve 26 to dump excess condensate when the level in the deaeratortank becomes excessive. From the deaerator tank boiler -feedwater flows through heater 2l to a pair of feedwater pumps 23 connected in parallel and .driven by turbines 29. The discharge from the feedwater pumps 28 goes through a heater 3l into a ieedwater return manifold 32 which is connected to all fourof the boilers II), Il, I2 and i3. Each boilermay be provided `with an individual feedwater control -indicated at lun to control supply of feedwater thereto from the header 32 to maintain the proper level therein.

Steam discharged from the turbine 2l is condensed in a condenser' and is pumped by a purnpii into a deaerator tank 35. The tank 35 contains a float 35 controlling a bypass valveY 31 similar to the valve 26. From the deaerator feedwater flows to a surge' tank 38 and through heaters 39 to feedwater pumpstl connected in parallel. The pumps il -are driven by turbines mand the discharge from said pump moves through a heater 43 into the feedwater return manifoldSZ.

With this system the ieedwater `pumps 28 pump into the headers?! a quantity of condensate proportional to the steam condensed in the condenser 22 and the pumps li-pump a quantity proportional to the steam condensed in condenser If the turbine il and condenser 22 are drawing steam from boilers I2 and I3 the pumps 28 will return-more condensate than is required by the boilers I l) and II'to maintain proper water flow-steam ow. balance andthe pumps `fil :will return. a correspondingly lesser amount than is required by the boilers I2 and. I3. The excess condensate pumped by the pumps 28 will therefore, flow from left to right in the header 32 into the boilers I2 and I3. The present invention provides a control system for the ieedwater pumps to maintain a proper steam flow-water ilow balance in the several boilers.

When the amount of4 steam owing from one bank of boilers toward the other through the header i6 is proportional to the amount of water flowing from the other bank toward the one bank through the header 52 the system is in balance.

The flow through the feed water pumps 28 is controlled by regulators 44 which are responsive to loading pressure supplied through lines i5 and to the flow through the respective pumps as measured by the drop across orifices d6. The regulators 154, as well as some of the other regulators to be referred to hereinafter, may be of the type more particularly described in Smoot Reissue Patent No. 16,507 or in OConnor 2,039,924 or may be transmitters or the type shown in the patent to Rosenberger, No. 2,431,200. The regulators control valves il which in turn control the supply of steam to the turbines 29 to control the pump.

As shown each regulator di! includes a balance beam SI urged in one direction by a diaphragm 82 whose opposite sides are connected to opposite sides of the restriction 4t. 83 responsive to the loading pressure in line 45 opposes the diaphragm S2 to modify the force exerted thereby in response to flow.

The beam 5I controls a bleed nozzle 34 supplied with air past a restriction 55 to produce a regulated pressure. The nozzle 84 is connected to a diaphragm 86 which urges the beam in the other direction to balance it and to the control valve lil to control it.

The pumps 4i are similarly controlled by regulators ill supplied with loading pressure through lines 49 and responsive to the ow through the associated pumps as measured across orifices 5I. The regulators 48 control valves 50 which control the supply of steam to the turbines 42. The

several loading pressures to the lines d5 and 49 are supplied thereto through transfer units 52, all of which may be identical. Each of the transfer units 52 may contain switch-over valves to supply automatically regulated loading pressure r these units are purely conventional units whose construction forms no part of the present invention, they are not illustrated in detail.

The automatic loading pressure to control the regulators 4B is supplied to the corresponding transfer units 52 through a pipe 5S leading from a relay valve 52 in a regulator 55 which may be of the type shown in the patent to OConnor No. 2,039,924. The relay valve as shown may be of conventional construction including a diaphragm I5!! dividing a hollow casing |55 and carrying a central valve pin E55 controlling a vent opening in the bottom of the casing. The lower part of the casing is supplied with pressure from a regulater 6l described hereinafter and is also connected through the transfer units 52 to the regulators lit. A spring I5? urges the diaphragm down and seats against a movable block connected to the beam 55.

When the block is moved downward by the beam |56 the pressure below the diaphragm will A loading diaphragm increase to a valve closer to that supplied from the regulator 61 and when the block moves up the pressure will decrease. The regulator 55 is jointly responsive to steam iiow and condensate flow through the headers in a direction from one bank of boilers to the other and may be corrected in response to an excess or deciency of condensate in one of the deaerator tanks. As shown, the regulator includes a balanced beam 56 pivoted intermediate its ends and urged in one direction by a diaphragm 57 which is supplied with loading pressure by a flowmeter or transmitter 58 responsive to the differential across an orifice 59 in the steam header I6 between the banks of boilers. The flowmeter or transmitter may be of the type disclosed in the patent to Rosenberger, No. 2,220,176. A second flowmeter or transmitter 5 I responsive to the differential across an orifice 62 in the feedwater return header 32 between the banks of boilers supplies a loading pressure to a diaphragm 63 which opposes the diaphragm 5l. In normal operation, as long as the steam now and feedwater ilow between the banks of boilers is in the correct proportion, the beam 56 will be balanced and the relay valve 54 will be kept in its normal position to transmit a loading pressure through the transfer units 52 to the regulators 4S.

In the event that an excessive amount of condensate should accumulate in one of the deaerator tanks it is necessary to make a correction to prevent dumping of condensate. For this purpose a transmitter 54 is provided responsive to the condensate level in the deaerator tank 24. The transmitter 64 is a conventional instrument as shown for example in the patent to Rosenberger, No. 2,431,200, and supplies loading pressure to a correcting diaphragm 55 acting on the weigh beam 55 which is pivoted intermediate its ends and opposing the diaphragm 53. The transmitter 5G as shown includes a balance beam 8l' urged in one direction by a diaphragm 88 connected to the lower part of the tank 2li to exert a force proportional to the water level in the tank. rllhe beam is urged in the opposite direction by a diaphragm 89 connected to a bleed nozzle SI which is supplied with air through a restriction 52. The nozzle is also connected to the diaphragm 65 to supply loading pressure thereto. When the level in the tank 24 is within the desired limits, the transmitter 54 will send out a steady loading pressure of a normal value. If the level in the tank 24 should increase above the desirable limits, the loading pressure sent out by the transmitter 54 will increase to increase the effect of the diaphragm This reduces the effective force of the diaphragm 63 tending to move the block I5 up to decrease the pressure supplied to the transfer units 52 and regulators d8 and has the same result as an insufficient flow or" feedwater from left to right in the header 32. As explained more fully hereinafter, this will reduce the speed of the pumps 4I and increase the speed of the pumps 28 to pump more water from the tank 24 so that the level therein will be reduced. Simultaneously less water will be pumped from tank 35 so that the level therein which will be too low under the conditions stated, will increase. Similarly, should the level fall too low in the tank 24, the loading pressure on diaphragm 65 will be decreased to increase the effect of the diaphragm 63 and increase the speed of pumps il while decreasing the speed of pumps 28. In this way the level in the tank 24, which is a measure Of the difference between the cuantity` of'` condensate/delivered ley-,fthe 1condenserrr22f. and pumpedifby; thefgpumpsr 2,8-, serves :to-.correct the system `fto prevented .lunbalancedfconditon therein:

Thefpressure tothe relay .-va.l-ve.5lli W-hohfisniodied by operation of the-regulator E5', istin-` eiect a master loading pressure supplied through a pipel 65 from afregulatorh 6l The regulatorv Sl contains a-.relay valve-:S8 supplied vv-ithairv from Aa ,suitable source ,pastA a rrestriction .-59 and sending out aemasterY loading pressure intoA the pipev 66 inaccordance :with the operationpf the regulator: The. regulator El vis controlled inaresponse to a; function of-therfeedwaterfreturn inn the `headerElZ. As. shown-apipe 'H connects the l header 32..\to,a diaphragm 93- in the regulator l southat the regulator. is. responsive. to the pressure-in the. header. The .diaphragm .93 urges a-balance beamed in. one ,direction and the beam controlsthe valveor. nozzle vto .produce the master 1 load-ingY pressure. The beam. .is balanced by a diaphragm Q5 ,whichis connected tothe valve orv nozzle t8 torbe responsive .to theloading pressure. In operation, the regulatortends to controltheother regulators. in` the y:system in a` mannerto vmaintain.theheaderpressure. confstant. .so that an adequate fsupply of feedvvater will. be available for al1 of. theboilers.

The loadingpressureforthe. regulators @il is. supplied by a-loW `balance relay which functions to maintain the totalflow through all of the feedwater pumps at'the. proper value andto proportion the flows through the separate pumps 23.. and 4l Sothatthe vproper amount ot condensate Will be pumpedby each.. The now balancerelay. as showndncludes a. balance .lever l2 urgediin one direction by` a. diaphragmv'i con-` nected to the pipe .631, to be vresponsive tothe.

lnasterrloading pressure.. The. leveris urged .in the oppositedirection by a pair ofdiaphragrns lll and 'l5 which act in parallel so Vthat their total force mustrbe such. as.to .ba-lance force. or the. Asshown, thel diaphragm .la is connected to the pipe 53 to be responsive tothe.

diaphragm 13.

master Loading pressure supplied to the regue. laters 58.

A nozzle 'lt is mounted adjacent the lever. l2v and is supplied With air through a restricted ori- Tice il 'so vthat the pressure back of the nozzlek Will'be a regulated loadingwpressure controlled by balance of the relay lever 'l2'. This regulated loading` pressure is supplied through a pipe l5) to the diaphragm 'idand through the lefthand' transfer units 521to .theregulators 1M,

In operation, vassuming that steam is iov/ing from the bank of boilers iland I3 to the turbine i? and that feedwater is flowing from left to right through .the header 32, if `the steamv flow from boilers irland i3 andfeedwater new to boilers providedin` the headers between each bank and the adjacent banks.

Ef the .pumps 2S should .be pumping insufficient i eedwater into the header 32 to maintain the desired balance, the flovvthrough header 321 past. tlleforiiice` 52 `will be too small `and the. loading i pressure supplied'todiaphragm St'wllbe too these @conditioner thesrelayvalvezill .fw-illlbe: adjusted. 'to .reduce :the Sloadinggpressure suppliedvto the regulatorsf 48s andothesef; regulators vt will y reduce the 4speed :ein the ffeedvvater-u pumps.- 4 I.. At the. same, ti1ne, thev pressure Toni diaphragmV 'lll fis reduced so. that rthefleverfllwll rocky clockwise toapproaelnthe?nozzlev'l,v This will: increase the loading,pressureginline lffto thetpoint where the diaphragm.'lf-willurebalance the lever T2, and `ati-the sanefv--timeswill increase the :loading pressures acting;k on f the regulatorsgv fill.r4 These regulators will open the valves 41, increasing the supply yofistearnyito;y the -turbines\A ZSrand :increas 'zing-.5.; the quantity; of-'lfeedwaterf pumped-1 by` the pumps:` 28..- lit-1 the;- system. '.is-zV properly adj usted', the-increase in the amount pumped by thefpumpsf 28;-andwthe deereasein the amount pumpedbv' the-pumps 4 I .I willfbe equal vf and -fwill be-.suicient to .restore 'fthe 1 propen steam flow: and ffeedwater flow balance: pumping. toovmuoh: softhat thev noi/vf past the orice 62 i too great... for.' proper. balance;l the operation willbe y-the reverse of i that deseribed.

condensate` inthe tank` 2.4 Willrtendreither to increase -or` decrease-beyond the desired .-values.-

Under f these. conditioner thee transmitten dmvill f operate the diaphragm Y'6 5ft@ adjust the regulator 55 so thateproper. balance inuthe.l ows is -re- Y stored.. It will .thus.:be.seen thatwith the present systerrr feedWate-r return balance isrnain.-l tainedl under all conditions' so-.fthat there. is `no :unnecessary dumpingfofi-.condensater While. one. embodiment. of, the. invention: has been.. illustrated'. andodescribedztin'. detail, be apparentV that .ma-ny changes might be. made therein andthe embodiment. shown. is for the purpose of illustration. only.` and =is;not tov be taken vas va/definition. of. ,the scope. ofA the invention,4 reference-.being `had -forthis ,purpose tothe appended. claims..4

What dis .claimed is:

1.,.A feedvvater control-system fora power plant including a.plur.ality of-` banks .of steam boilersA interoonnectedubv steam and, feedwater headers for., flow.. of steam.: and ,feedwater. between said banks, acondenserfforeach bank of boilers "adapte ed .tobe connected to ,a :steam consuming unit -to receive. steam..therefromland a Vfeed-water. pump connectingeach1 condenser.. to vits associated .set

of .boilersfsaid .control `:systemA comprising control.

mansion eaohieedxvater. pump,. a owmeter connectedto therstearn. header formeasuring flow of. steam. throughthevsteam .header f rom one bank of boilenseto Athe. other, a .flowmeter connected to the. .feedwater header for measuring. the now of Weiten through theieedwateryheader from: one bankof .boilers to the-other,- a,balance control devicelconnected Xto'` thecontrol means to` maintaina predeterminedflow balance. rtlfir-ough the feedvvateru. pumps;v .andrneans ,j ointlyV responsive to.theilowmeters. actingeonthe balance control device, to .verveeld flovabalance.

2,- .Afeeelwater` control system .fora power `plant including .afplurality of. banksof,` steam boilers interonnected fior.; flow rofsteain. :and eedwater therebetween., a... condenser. for. each bank.- of boilers adapted '.to.,b.eoonnected to astearn. cone suming, unit... to receive steam, therefrom, and. a feedwater.pnmponnectingreaoh condenser to, its. assocated.. set. oifboilersmsaid control .systemioomprising v. controle, means. for, each. ieedwater pump aerlowmeter.fonrneasuring :,low.. of .steam If, the systernf shoulda ,getr out of adjustment or should not be -perfeotlyf'adjusted,. .thelevel-:ot-

it will 1 through the steam header from one bank of boilers to the other, a flowmeter connected to the feedwater header for measuring flow of water through the feedwater header from one bank of boilers to the other, a balance control device connected to the control means to maintain a predetermined flow balance through the feedwater pumps, means jointly responsive to the owmeters acting on the balance control device to vary said flow balance, and means responsive to the quantity of condensate delivered by one of said condensers to vary the effect of the last named means.

3. A feedwater control system for a power plant including a plurality of banks of steam boilers interconnected by steam and feedwater headers for flow of steam and feedwater between said banks, a condenser for each bank of boilers adapted to be connected to a steam consuming unit to receive steam therefrom, and a feedwater pump connecting each condenser to its associated set of boilers, said control system comprising control devices of each feedwater pump, a flow meter connected to the steam header for measuring the flow of steam through the steam header from one bank of boilers to the other, a flow meter connected to the feedwater header for measuring the flow of feedwater through the feedwater header from one bank of boilers to the other, a regulator jointly responsive to the new meters to control one of the control devices, and a ow balance relay responsive to the regulator to control another of the control devices in a direction inverse to the direction of control of said one of the control devices.

4. A feedwater control system for a power plant including a plurality of banks or steam boilers interconnected for now of steam and feedwater therebetween, a condenser for each bank of boilers adapted to be connected to a steam consuming unit to receive the steam therefrom, and a ieedwater pump connecting each condenser to its associated set of boilers, said control system comprising control devices for each feedwater pump, a flowmeter for measuring the ow of steam through the steam header from one bank of boilers to the other, a flowmeter connected to the feedwater header for measuring the flow of feedwater through the feedwater header from one bank of boilers to the other, means responsive to the quantity of condensate delivered by one of the condensers, a regulator jointly responsive to the last named means and to one of the flowineters to control one of the control devices, and a flow balance relay responsive to the regulator to control another of the control devices inversely to said control of said one of the control devices.

5. A feedwater control system for a power plant including a plurality of banks of steam boilers interconnected for ow of steam and feedwater therebetween, a condenser for each bank of boilers adapted to be connected to a steam consuming unit to receive steam therefrom, and a feedwater pump connecting each condenser to its associated set of boilers, said control system comprising control devices for each feedwater pump, a ilowmeter for measuring the flow of steam through the steam header from one bank of boilers to the other, a owmeter connected to the feedwater header for measuring the flow of feed- Water through the feedwater header from one bank of boilers to the other, a regulator responsive to the pressure of the feedwater simultaneously to control both of the control devices, a regulator jointly responsive to the flowmeters to modify the control of one of the control devices by the rst named regulator, and a flow balance relay responsive to the last named regulator which is jointly responsive to the owmeters to modify the control of another of the control devices by the rst named regulator.

6. A feedwater control system for a power plant including a plurality of banks of steam yboilers, interconnected for flow of steam and feedwater therebetween, a condenser for each bank of boilers adapted to be connected to a steam consuming unit to receive steam therefrom, and a feedwater pump connecting each condenser to its associated set of boilers, said control system comprising a pair of control devices for the feedwater pumps respectively, a iiowmeter for measuring the flow of steam through the steam header from one bank of boilers to the other, a ilowmeter connected to the feedwater header for measuring the iow of feedwater through the feedwater header from one bank of boilers to the other, a regulator responsive to the pressure of "ie feedwater simultaneously to control both of the control devices, means responsive to the `quantity of condensate delivered by one of the condensers, a second regulator jointly responsive to the flowmeters and the last named means to modify the control of one of the control devices by the first named regulator, and a flow balance relay responsive to the second regulator to modify control of the other of the control devices by the rst named regulator.

T. A feedwater control system for a power plant including a plurality of banks of steam boilers interconnected by steam and feedwater headers for flow of steam and feedwater between said banks, a condenser for each bank of boilers adapted to be connected to a steam consuming unit to receive steam therefrom, and a feedwater pump connecting each condenser to its associated set of boilers, said control system comprising a control device for each of the feedwater pumps, owmeters connected to the headers and responsive respectively to steam ow and feedwatcr flow through the headers from one bank of boilers to the other to develop loading pressures proportional to functions of the flows, a flow balance relay including means to produce a force in one direction and a pair of units to produce balancing forces in the opposite dir ction, a regulator jointly responsive to the iiowmeters to regulate a loading force acting on one of the units and controlling one of the devices, and means responsive to unbalance of the relay to produce a second loading force acting on the other unit and controlling the other control device.

8. A feedwater control system for a power plant including a plurality of banks of steam boilers interconnected by steam and feedwater headers for flow of steam and feedwater between said banks, a condenser for each bank of boilers adapted to be connected to a steam consuming unit to receive steam therefrom, and a feedwater pump connecting each condenser to its associated set of boilers, said control system comprising a control device for each of the feedwater pumps, lowmeters connected to the headers and responsive respectively to steam new and feedwater now through the headers from one bank of boilers to the other to develop loading pressures proportional to functions of the flows, a flow balance relay including means to produce a force in one direction and a pair of units to produce balancing forces in the opposite direction, means responsive to the difference between the quantity of condensate discharged by one of the condenser-s and the quantity pumped by the connected pump, a regulator jointly responsive to the owmeters and the last named means to regulate a loading force acting on one of the units and controlling one of the control devices, and means responsive to unbalance of the relay to produce a second loading force acting on the other unit and the other of the control devices.

9. A feedwater control system for a power plant including a plurality of banks of steam boilers interconnected by steam and feedwater headers for iiow of steam and feedwater between said banks, a condenser for each bank of boilers adapted to be connected to a steam consuming unit to receive steam therefrom, and a feedwater pump connecting each condenser to its associated set of boilers, said control system comprising a control device for each of the feedwater pumps, iiowmeters connected to the headers and responsive respectively to steam flow and feedwater flow through the headers from one bank of boilers to the other to produce loading pressures proportional to functions of the ows, a, flow balance relay including a rst pressure responsive unit to produce a force in one direction and second and third pressure responsive units to produce balancing forces in the opposite direction, a regulator responsive to the pressure of the feedwater to produce a loading pressure acting on the first unit, a regulator jointly responsive to the flowmeters and receiving loading pressure from the first regulator to modify the loading pressure and supply the modified pressure to the second unit and one of the control devices, and means responsive to unbalance of the relay to produce a loading pressure acting on the third unit and the other of the control devices.

10. A feedvvater control system for a power plant including a plurality of banks of steam boilers interconnected by steam and feedwater headers for iiow of steam and feedwater between said banks, a condenser for each bank of boilers adapted to be connected to a steam consuming unit to receive steam therefrom, and a feedwater pump connecting each condenser to its associated set of boilers, said control system comprising a, control device for each of the feedwater pumps, fiowmeters connected to the headers and responsive respectively to steam flow and feedwater flow through the headers from one bank of boilers to the other to produce loading pressures proportional to functions of the flows, a flow balance relay including a first pressure responsive unit to produce a force in one direction and second and third pressure responsive units to produce balancing forces in the opposite direction, a, regulator responsive to the pressure of the feedwater to produce a loading pressure acting on the first unit, means responsive to the difference between the quantity of condensate delivered by one of the condensers and the quantity pumped by the connected pump, a regulator jointly responsive to the owmeters and the last named means and receiving loading pressure from the rst regulator to modify the loading pressure and supply the modied pressure to the second unit and one of the control devices, and means responsive to unbalance of the relay to produce a loading pressure acting on the third unit and the other of the control devices.

11. A feedwater control system for a power plant including a plurality of banks of steam boilers, steam and water headers connected to all of the boilers, and a plurality of feedwater pumps connected to spaced points on the feedwater header, said control system comprising control means for each feedwater pump, a iiowmeter in the steam header to measure the flow of steam through the steam header from the point of connection of one bank of boilers thereto to the point of connection of the other bank of boilers thereto, a flow meter in the water header to measure the flow of water therethrough between said spaced points thereon, a balance control device connected to the control means to maintain a predetermined flow balance through the feedwater pumps, and means jointly responsive to the flowmeters acting on the balance control device to vary said flow balance.

12. A feedwater control system for a power plant including a plurality of banks of steam boilers, steam and water headers connected to all of the boilers, a plurality of condensers, and a feedwater pump connecting each of the condensers to the Water header, said control system comprising a control device for each of the feedwater pumps, control means responsive to the quantity of condensate delivered by one of the condensers to produce a controlling force, means responsive to said controlling force to control the control device for one of the feedwater pumps, and a relay responsive to said last named means to control the control device for another of the feedwater pumps inversely to the control of the rst named control device.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,860,364 La Mont May 31, 1932 1,898,020 Roe Feb. 21, 1933 1,964,773 Smoot July 3, 1934 1,975,086 Dickey Oct. 2, 1934 2,371,443 Hillier Mar. 13, 1945 

